Optical measuring systems exist for making more accurate distance measurements. One traditional type of system is the narrowband ranging system. This system emits one or more modulated optical signals that produce reflections on an incident target. The system captures the reflections and determines the distance to the target based on phase shifts detected in the captured reflections. These systems typically require the use of an expensive high precision receiver, such as an avalanche gain photodiode. The performance of these systems can also erode as the signal to noise ratio falls. This can be a significant drawback, because environmental conditions in the working area can provide substantial signal attenuation.
Another traditional type of system is the wideband pulsed system. This system also emits one or more optical signals that produce reflections on an incident target. The system captures the reflections and measures the round trip signal delay to obtain the distance to the target. The system determines the time difference between the time a signal pulse departs the system and the time that the system receives a reflection of the pulse. Traditional systems identify pulse departure and arrival through threshold detection i.e. comparing the signals to a threshold level. One typical technique is half-maximum detection, which establishes a reference threshold based on the peak intensity of the signal pulses. Unfortunately, this technique does not operate well in low signal to noise ratio environments. The system has difficulty establishing a consistent detection point, because the low signal to noise ratio increases estimation errors in the measurement of signal amplitude. Challenges also arise when trying to measure time delay between signal pulses. When an asynchronous clock is employed to measure the time between pulses, significant inaccuracies can occur unless the system employs measurement intervals with impractically long durations. In order to avoid such measurement intervals, the system can employ expensive high-speed components with substantial power consumption.
Co-pending U.S. patent application Ser. No. 10/414,440 entitled Distance Measuring Device, filed Apr. 15, 2003, describes a number of embodiments of a “time of flight” distance measuring device. The device emits a beam that reflects on the surface of an object. The measurement device captures the return beam and determines the distance to the object, based on the time of flight of the beam from transmission to capture by the measurement device.
One implementation of the measurement device enhances accuracy by deriving feedback reference pulses from pulses in the emitted beam and injecting them into the device's receive path. This creates a receive waveform that includes one or more feedback reference pulses in the emitted beam and corresponding return pulses in the return beam. This enables the measurement device to directly measure time delay between a return pulse and a reference pulse that lead to the generation of the return pulse.
A variety of problems are encountered in optical ranging systems when the range being measured is decreased. For a beam that is contained within the field of view of the receiver, there is a 1/R2 relationship between the return signal strength and the distance (where R is the distance of the reflected beam from the detector). To obtain measurements at short distances, a large dynamic range in the reflected beam is required. This adds complexity to signal detection and processing circuitry. In systems with separate transmit and receive apertures, decreasing the distance measured eventually causes a drop off in signal strength at the returned detector. This signal drop off occurs as the received signal moves off of the active surface of the detector. For time of flight ranging systems requiring wide detection bandwidths, a large detector is not feasible. This means other techniques must be available to prevent a short range signal dropout. In time of flight range finders incorporating pulsed laser diodes, such as that described in the co pending application cited above, there is often a dependence of the time behavior of the return signal on the distribution of the return signal falling on the detector. If only a portion of the return signal falls on the detector, as experienced at short distances, changes in the pulse shape can result.
Aspects of the present invention can be accomplished using hardware, software, or a combination of both hardware and software. The software used for the present invention is stored on one or more processor readable storage media including hard disk drives, CD-ROMs, DVDs, optical disks, floppy disks, tape drives, RAM, ROM or other suitable storage devices. In alternative embodiments, some or all of the software can be replaced by dedicated hardware including custom integrated circuits, gate arrays, FPGAs, PLDs, and special purpose computers. In one embodiment, software implementing the present invention is used to program one or more processors. The processors can be in communication with one or more storage devices, peripherals and/or communication interfaces.